By: Uwe
Re-posted from: https://ufechner7.github.io/2022/08/12/filtering-and-browsing-datasets.html
Introduction
The basic work flow of data analysis is filtering a large data set to extract the interesting aspects and to browse and/or plot the result. In this post I will give a basic example for filtering and browsing of data, plotting deserves a blog post on its own.
Creating a test project
For trying out a new package and/or example it is always good to create a new project first.
When you followed my first post you only have to add the package TerminalPager.
For example using the following commands:
mkdir can
cd can
julia --project="."
And then install the required packages:
julia> using pkg
julia> pkg"add InMemoryDatasets"
julia> pkg"add TerminalPager"
Now quit julia with
julia --project -t auto
This uses the set of packages we just installed and starts julia using all available threads. This is useful when handling large data sets (millions of messages).
Creating a sample data set
To get a realistic impression of the performance we now create a sample data set with 100k entries. We use again a CAN bus log file as example, this time with m=8 data columns, one byte per column. For the address column, which uses two bytes we use a range of 46 different, randomly choosen values, starting with 0x101.
using InMemoryDatasets, DLMReader, Printf
function demo_data()
n = 100000
m = 8
time = 0.1:0.1:n*0.1
addr = rand(Int16(0x101):Int16(0x12e), n)
data = (Symbol("d", i) => rand(UInt8, n) for i in 1:m)
ds = Dataset(;time, addr, data...)
ds.addr[5] = missing
ds
end
ds = demo_data()
If we now print the data set in the julia console, only the first and the last values are shown:
julia> ds = demo_data()
100000×10 Dataset
Row │ time addr d1 d2 d3 d4 d5 d6 d7 d8
│ identity identity identity identity identity identity identity identity identity identity
│ Float64? Int16? UInt8? UInt8? UInt8? UInt8? UInt8? UInt8? UInt8? UInt8?
────────┼────────────────────────────────────────────────────────────────────────────────────────────────────
1 │ 0.1 264 127 5 247 185 212 216 171 147
2 │ 0.2 278 127 239 183 48 127 98 27 44
3 │ 0.3 301 6 16 159 201 225 95 196 51
⋮ │ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮
99999 │ 9999.9 294 95 185 15 205 178 64 13 31
100000 │ 10000.0 285 86 224 79 214 248 39 34 147
99995 rows omitted
Formating the data
For CAN bus messages usually the hexadecimal data format is used. To see the numbers in hex format we need to define two functions, one that is formating the data bytes with two hex digits, called hex2(n) and one that formats the address column with four hex digits, called hex4(n). To save space we want to display missing values with the string “–”.
function hex2(n)
if ismissing(n) return "--" end
string(n, base=16, pad=2)
end
function hex4(n)
if ismissing(n) return "--" end
string(n, base=16, pad=4)
end
The setformat! function can be used to set the format of a column of a dataset. The exclamation mark indicates that this function modifies the first argument. The function Symbol can be used to create symbols, composed of a letter and a number to be used in a loop.
setformat!(ds, :addr => hex4)
for i in 1:8
setformat!(ds, Symbol("d", i) => hex2)
end
Filtering the data on one column
Let us assume that the devices with the address 0x103, 0x106 and 0x109 are output devices, and we want to filter all the messages sent to the output devices. For complex filter conditions a function that returns true or false is needed. Missing values must be handled correctly.
# CAN bus devices with a given set of addresses are output devices
function isoutput(addr)
if ismissing(addr) return false end
addr in [0x103, 0x106, 0x109]
end
Finally we can apply the filter function, which requires three parameters, the input dataset, the column for applying the filter function and – as named parameter – the filter function.
# create the dataset msg_out with all messages sent to output devices
msg_out = filter(ds, :addr, by=isoutput)
Filtering the data on two columns
Lets assume that in addition to apply a filter on the column addr we are only interested in the data from t=1000s to t=3000s. Then we need a second filter function:
function ininterval(time)
if ismissing(time) return false end
time >= 1000.0 && time <= 3000.0
end
and pass arrays of the column names and of the column filter functions to the dataset filter function.
msg_out_ininterval = filter(ds, [:time, :addr], by=[ininterval, isoutput])
Browsing through the output data
The filtered data set msg_out is still too big to print it on one screen, so we need a way to browse through the result set. I am using the following function to do that:
function browse(ds)
io = IOContext(IOBuffer(), :color => true);
show(io, ds, show_row_number=false, eltypes=false)
pager(String(take!(io.io)), frozen_rows=3)
end
The option :color => true allows the use of formatting, e.g. the header will be in bold.
The options show_row_number=false and eltypes=false suppress the row numbers and the row with the column types.
You can now browse through the output messages with the command:
browse(msg_out)
It will look like this (well, with the header in bold):
6475×10 Dataset
time addr d1 d2 d3 d4 d5 d6 d7 d8
──────────────────────────────────────────────
8.2 0109 17 d6 13 e2 60 14 a4 8f
9.9 0103 45 14 61 a3 6f 18 e7 35
12.1 0109 7b 16 43 65 e9 f2 87 99
12.8 0106 87 c0 46 f1 4e 49 ad 74
13.8 0106 bc 88 f1 0a a9 c4 99 ef
14.1 0106 63 bb 83 96 8f 56 9a 99
14.8 0106 f7 94 2b 01 b3 45 12 3a
15.3 0109 3e 74 04 16 15 93 5f 0c
15.6 0103 7b e6 53 c7 0d 89 6b f3
15.7 0109 8e db d2 30 c1 b0 32 fa
16.0 0106 5e 52 da 97 74 81 e6 b8
:
You can scroll through the dataset with the cursor keys and the page up and page down keys. You can also search for a string with the slash key, then typing the search term and then <ENTER>. To finish browsing press q.
As you can see, all the data is nicely formatted as hexadecimal numbers.
Further reading
To learn more about the filtering options, have a look at Filter observations.
Acknowledgements
Thanks to James D Foster for providing the browse() function and the developers of InMemoryDataframes for their nice package.
